1. Field of the Invention
The present invention relates to an exhaust gas-purifying apparatus for purifying exhaust gases including particulates, such as those emitted from diesel engines. More particularly, it relates to an exhaust gas-purifying apparatus used in exhaust systems in which a liquid reducing agent is supplied into the exhaust gases intermittently.
2. Description of the Related Art
Regarding gasoline engines, harmful components in the exhaust gases have been reduced securely by the strict regulations on the exhaust gases and the technological developments capable of coping with the strict regulations. However, regarding diesel engines, the regulations and the technological developments have been advanced less compared to those of gasoline engines because of the unique circumstances that the harmful components are emitted as particulates (i.e., particulate matters, such as carbonaceous fine particles, sulfuric fine particles like sulfates, and high-molecular-weight hydrocarbon fine particles, hereinafter collectively referred to as “PMs”).
As exhaust gas-purifying apparatuses having been developed so far for diesel engines, the following have been known. For example, the exhaust gas-purifying apparatuses can be roughly divided into trapping (or wall-flow) exhaust gas-purifying apparatuses and open (or straight-flow) exhaust gas-purifying apparatuses. Among these, clogged honeycomb structures made from ceramic (i.e., diesel PMs filters, hereinafter referred to as “DPFs”) have been known as one of the trapping exhaust gas-purifying apparatuses. In the DPFs, the honeycomb structures are clogged at the opposite openings of cells in a checkered manner alternately, for instance. The DPFs comprise inlet cells clogged on the downstream side of the flow of exhaust gases, outlet cells neighboring the inlet cells and clogged on the upstream side of the flow of the exhaust gases, and cellular walls demarcating the inlet cells and the outlet cells. The DPFs inhibit the emission of PMs by filtering the exhaust gases with the pores of the cellular walls to collect PMs.
The pressure loss, however, increases as PMs deposit on the DPFs. Accordingly, it is needed to regularly remove deposited PMs to recover the DPFs by certain means. Hence, when the pressure loss increases, deposited PMs have been burned with burners or electric heaters conventionally, thereby recovering the DPFs. However, in this case, the greater the deposition of PMs is, the higher the temperature increases in burning deposited PMs. Consequently, there might arise cases that the DPFs are damaged by thermal stress resulting from such burning.
Hence, continuously regenerative DPFs have been developed recently. For example, in one of the continuously regenerative DPFs, a coating layer comprising alumina is formed on the surface of the cellular walls of the DPF, and a catalytic ingredient such as platinum (Pt) is loaded on the coating layer. In accordance with the continuously regenerative DPFs, since the collected PMs are oxidized and burned by the catalytic reaction of the catalytic ingredient, it is possible to regenerate the DPFs by burning PMs simultaneously with or successively after collecting PMs. Moreover, since the catalytic reaction occurs at relatively low temperatures, and since PMs can be burned when they are collected less, the continuously regenerative DPFs produce an advantage that the thermal stress affecting the DPFs is so less that the DPFs are inhibited from being damaged.
Japanese Unexamined Patent Publication (KOKAI) No. 9-173,866 discloses such a filter catalyst. For example, the filter catalyst is made by forming a porous coating layer composed of first activated alumina whose particle diameter is larger than an average pore diameter of pores in the cellular walls, coating the inside of the pores with second activated alumina whose particle diameter is smaller than an average pore diameter of the pores, and further loading a catalytic ingredient thereon. In accordance with the filter catalyst, it is possible to make the pressure loss lower while enlarging the specific surface area of the porous coating layer.
Moreover, Japanese Unexamined Patent Publication (KOKAI) No. 6-159,037 discloses a filter catalyst which is made by further loading an NOx sorbing member on the porous coating layer. Thus, the NOx sorbing member sorbs NOx therein so that it becomes possible to purify the sorbed NOx by reduction by spraying a reducing agent, such as light oil.
However, in filter catalysts provided with a coating layer on which a catalytic ingredient and an NOx sorbing member are loaded, the forming amount of the coating layer is limited in view of making the pressure loss lower. Accordingly, the loading amount of the catalytic ingredient should be made less inevitably, because the catalytic ingredient should be loaded in such a highly dispersed manner in order to suppress the granular growth at high temperatures. Consequently, there arises a problem that the resulting filter catalysts might lack the PMs and NOx purifying performance. Moreover, in the case that low-temperature exhaust gases keep flowing into the filter catalysts, the resultant filter catalysts might suffer from the problem of the enlarged pressure loss, because they might exhibit such a low PMs oxidizing activity that PMs deposit in a large amount to clog the cells.
Hence, Japanese Patent Application No. 2001-212,506 (now published as Japanese Unexamined Patent Publication (KOKAI) No. 2002-115,524), Japanese Unexamined Patent Publication (KOKAI) No. 9-53,442 and Japanese Patent Application No. 11-5,285 (now published as Japanese Unexamined Patent Publication (KOKAI) No. 2000-204,940) disclose exhaust gas-purifying apparatuses in which a straight-flow structure oxidizing catalyst or NOx sorbing-and-reducing catalyst and a filter catalyst are disposed in series. When a straight-flow structure catalyst is used with a filter catalyst in combination, it is possible to improve the purifying performance of the resulting exhaust gas-purifying apparatus without enlarging the pressure loss. Moreover, when such a straight-flow structure catalyst is disposed on an upstream side of the flow of exhaust gases with respect to a filter catalyst, it is possible not only to upgrade the PMs oxidizing performance of the resultant exhaust gas-purifying apparatus, but also to suppress the increment of the pressure loss due to the clogging, because the purifying reaction of the upstream-side straight-flow structure catalyst increases the temperature of exhaust gases.
Incidentally, a system has been proposed in which a liquid reducing agent, such as light oil, is supplied into exhaust gases intermittently as a reducing agent in order to improve the NOx reducing activity, and is about to come into practical use. However, when the above-described exhaust gas-purifying apparatus, which comprises a straight-flow structure oxidizing catalyst or NOx sorbing-and-reducing catalyst and a filter catalyst disposed in series, is applied to the system, the liquid reducing agent flows directly into one of the catalysts disposed on the most upstream side of the flow of exhaust gases. As a result, a drawback might arise in that the most-upstream-side catalyst cannot demonstrate the catalytic activity fully and accordingly the considerable amount of the catalytic ingredient loaded thereon has been wasted.